The invention concerns an amplifier for electric signals obtained by means of a photoelectric transducer. The amplifier contains an operational amplifier in series with the photoelectric transducer and operates as a function of the intensity of illumination acting on the transducer. Amplifiers of this type are employed particularly in exposure measurements in connection with photographic cameras.
The illumination intensity on the photoelectric transducer may vary by several orders of magnitude in its absolute level. In an analogy to acoustics, it is possible to speak in terms of a large dynamic range or large dynamics. In photography, for example, in the case of an exposure meter, a brightness range of 4-5 orders of magnitude is employed. This range of brightness is related to the level of steady light. With other optical instruments, e.g., range finders, an alternating light signal is of interest; this may be superposed upon the steady light signal and may be significantly smaller in its amplitude, measured for example as the peak-to-peak value in the case of a sinusoidal signal, than the steady light signal.
The photoelectric transducers, which convert light into electric current, in accordance with the present state of the art, consist almost exclusively of semiconductor photodiodes. The currents supplied by these receivers are proportional over more than eight orders of magnitude to the light incident upon them, when operated in a conducting state, i.e., when the voltage on them is kept very small. The latter condition is readily satisfied with operational amplifiers which perform current-voltage transformation.
However, within the voltage range only about four orders of magnitude may be processed, because the output voltage of the operational amplifiers is limited upwardly by the supply voltage. Downwardly, small voltages are submerged in noise; this is a characteristic of all operational amplifiers. In a known amplifier circuit, voltage proportional to the incident light on the photoelectric transducer may be taken off at the output of said circuit, but only with a dynamic range of about four orders of magnitude. In cases where only the alternating signal is of interest and the latter is smaller by a factor of 100 than the steady light signal, the result is that only a dynamic range of two orders of magnitude is available.
Considering further that, as is known, the feedback resistance present in the feedback branch of the operational amplifier must be as high as possible for purposes of noise optimization (e.g., 1 GOhm), it may be readily seen that only a very narrow dynamic range remains. Otherwise, the amplifier is not optimum with respect to noise.
It is also known to provide in the feedback branch of an operational amplifier an element with a nonlinear, e.g., exponential, characteristic. The latter results in the voltage at the output of the amplifier becoming a logarithmic measure of the light incident upon the photoelectric transducer. The disadvantage of this form of design resides in the fact that the upper angular frequency of the transfer range varies with the level of steady light. In the case of low illumination intensities, the signal frequencies of interest are thus uncontrollably cut. In addition, the alternating voltage at the output of the amplifier is fundamentally distorted because of the exponential characteristics. This is particularly apparent with high amplitudes.